Backward-compatible long training sequences for wireless communication networks

ABSTRACT

A network device for generating an expanded long training sequence with a minimal peak-to-average ratio. The network device includes a signal generating circuit for generating the expanded long training sequence. The network device also includes an Inverse Fourier Transform for processing the expanded long training sequence from the signal generating circuit and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The expanded long training sequence and the optimal expanded long training sequence are stored on more than 52 sub-carriers.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

The present application is a CONTINUATION of U.S. application Ser. No.12,684,650, filed Jan. 8, 2010, now issued U.S. Pat. No. 7,990,842,which is a CONTINUATION OF U.S. application Ser. No. 11/188,771, filedJul. 26, 2005, now issued U.S. Pat. No. 7,646,703. Said U.S. applicationSer. No. 11/188,771 makes reference to, claims priority to and claimsbenefit from U.S. Application No. 60/591,104, filed Jul. 27, 2004; andU.S. Application No. 60/634,102, filed Dec. 8, 2004. Theabove-identified applications are hereby incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wireless communicationsystems and more particularly to long training sequences of minimumpeak-to-average power ratio which may be used by legacy systems.

2. Description of the Related Art

Each wireless communication device participating in wirelesscommunications includes a built-in radio transceiver (i.e., receiver andtransmitter) or is coupled to an associated radio transceiver. As isknown to those skilled in the art, the transmitter typically includes adata modulation stage, one or more intermediate frequency stages, and apower amplifier. The data modulation stage converts raw data intobaseband signals in accordance with a particular wireless communicationstandard. The intermediate frequency stages mix the baseband signalswith one or more local oscillations to produce RF signals. The poweramplifier amplifies the RF signals prior to transmission via an antenna.

The receiver is typically coupled to the antenna and includes a lownoise amplifier, one or more intermediate frequency stages, a filteringstage, and a data recovery stage. The low noise amplifier receives, viathe antenna, inbound RF signals and amplifies the inbound RF signals.The intermediate frequency stages mix the amplified RF signals with oneor more local oscillations to convert the amplified RF signal intobaseband signals or intermediate frequency (IF) signals. The filteringstage filters the baseband signals or the IF signals to attenuateunwanted out of band signals to produce filtered signals. The datarecovery stage recovers raw data from the filtered signals in accordancewith a particular wireless communication standard.

Different wireless devices in a wireless communication system may becompliant with different standards or different variations of the samestandard. For example, 802.11a an extension of the 802.11 standard,provides up to 54 Mbps in the 5 GHz band. 802.11b, another extension ofthe 802.11 standard, provides 11 Mbps transmission (with a fallback to5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11g, another extension ofthe 802.11 standard, provides 20+Mbps in the 2.4 GHz band. 802.11n, anew extension of 802.11, is being developed to address, among otherthins, higher throughput and compatibility issues. An 802.11a compliantcommunications device may reside in the same WLAN as a device that iscompliant with another 802.11 standard. When devices that are compliantwith multiple versions of the 802.11 standard are in the same WLAN, thedevices that are compliant with older versions are considered to belegacy devices. To ensure backward compatibility with legacy devices,specific mechanisms must be employed to insure that the legacy devicesknow when a device that is compliant with a newer version of thestandard is using a wireless channel to avoid a collision. Newimplementations of wireless communication protocol enable higher speedthroughput, while also enabling legacy devices which might be onlycompliant with 802.11a or 802.11g to communicate in systems which areoperating at higher speeds.

Devices implementing both the 802.11a and 802.11g standards use anorthogonal frequency division multiplexing (OFDM) encoding scheme. OFDMis a frequency division multiplexing modulation technique fortransmitting large amounts of digital data over a radio wave. OFDM worksby spreading a single data stream over a band of sub-carriers, each ofwhich is transmitted in parallel. In 802.11a and 802.11g compliantdevices, only 52 of the 64 active sub-carriers are used. Four of theactive sub-carriers are pilot sub-carriers that the system uses as areference to disregard frequency or phase shifts of the signal duringtransmission. The remaining 48 sub-carriers provide separate wirelesspathways for sending information in a parallel fashion. The 52sub-carriers are modulated using binary or quadrature phase shift keying(BPSK/QPSK), 16 Quadrature Amplitude Modulation (QAM), or 64 QAM.Therefore, 802.11a and 802.11g compliant devices use sub-carriers −26 to+26, with the 0-index sub-carrier set to 0 and 0-index sub-carrier beingthe carrier frequency. As such, only part of the 20 Mhz bandwidthsupported by 802.11a and 802.11g is use.

In 802.11a/802.11g, each data packet starts with a preamble whichincludes a short training sequence followed by a long training sequence.The short and long training sequences are used for synchronizationbetween the sender and the receiver. The long training sequence of802.11a and 802.11g is defined such that each of sub-carriers −26 to +26has one BPSK consellation point, either +1 or −1.

There exists a need to create a long training sequence of minimumpeak-to-average ratio that uses more sub-carriers without interferingwith adjacent channels. The inventive long trains sequence with aminimum peak-to-average power ratio should be usable by legacy devicesin order to estimate channel impulse response and to estimate carrierfrequency offset between a transmitter and a receiver.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a networkdevice for generating an expanded long training sequence with a minimalpeak-to-average ratio. The network device includes a signal generatingcircuit for generating the expanded long training sequence. The networkdevice also includes an Inverse Fourier Transform for processing theexpanded long training sequence from the signal generating circuit andproducing an optimal expanded long training sequence with a minimalpeak-to-average ratio. The expanded long training sequence and theoptimal expanded long training sequence are stored on more than 52sub-carriers.

According to another aspect of the invention, there is provided anetwork device for generating an expanded long training sequence with aminimal peak-to-average ratio. The network device includes a signalgenerating circuit for generating the expanded long training sequence.The network device also includes an Inverse Fourier Transform forprocessing the expanded long training sequence from the signalgenerating circuit and producing an optimal expanded long trainingsequence with a minimal peak-to-average ratio. The expanded longtraining sequence and the optimal expanded long training sequence arestored on more than 56 sub-carriers.

According to another aspect of the invention, there is provided anet-work device for generating an expanded long training sequence with aminimal peak-to-average ratio. The network device includes a signalgenerating circuit for generating the expanded long training sequence.The network device also includes an Inverse Fourier Transform forprocessing the expanded long training sequence from the signalgenerating circuit and producing an optimal expanded long trainingsequence with a minimal peak-to-average ratio. The expanded longtraining sequence and the optimal expanded long training sequence arestored on more than 63 sub-carriers.

According to another aspect of the invention, there is provided a methodfor generating an expanded long training sequence with a minimalpeak-to-average ratio. The method includes the steps of generating theexpanded long training sequence and producing an optimal expanded longtraining sequence with a minimal peak-to-average ratio. The method alsoincludes the step of storing the expanded long training sequence and theoptimal expanded long training sequence on more than 52 sub-carriers

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention thattogether with the description serve to explain the principles of theinvention, wherein:

FIG. 1 illustrates a communication system that includes a plurality ofbase stations, a plurality of wireless communication devices and anetwork hardware component;

FIG. 2 illustrates a schematic block diagram of a processor that isconfigured to generate an expanded long training sequence;

FIG. 3 is a schematic block diagram of a processor that is configured toprocess an expanded long training sequence;

FIG. 4 illustrates the long training sequence that is used in 56 activesub-carriers; and

FIG. 5 illustrates the long training sequence that is used in 63 activesub-carriers.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the preferred embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a communication system 10 that includes a pluralityof base stations and/or access points 12-16, a plurality of wirelesscommunication devices 18-32 and a network hardware component 34.Wireless communication devices 18-32 may be laptop computers 18 and 26,personal digital assistant hosts 20 and 30, personal computer 24 and 32and/or cellular telephone 22 and 28. Base stations or access points12-16 are operably coupled to network hardware 34 via local area networkconnections 36, 38 and 40. Network hardware 34, for example a router, aswitch, a bridge, a modem, or a system controller, provides a wide areanetwork connection for communication system 10. Each of base stations oraccess points 12-16 has an associated antenna or antenna array tocommunicate with the wireless communication devices in its area.Typically, the wireless communication devices register with a particularbase station or access point 12-14 to receive services fromcommunication system 10. Each wireless communication device includes abuilt-in radio or is coupled to an associated radio. The radio includesat least one radio frequency (RF) transmitter and at least one RFreceiver.

The present invention provides an expanded long training sequence ofminimum peak-to-average power ratio and thereby decreases powerback-off. The inventive expanded long training sequence may be used by802.11a or 802.11g devices for estimating the channel impulse responseand by a receiver for estimating the carrier frequency offset betweenthe transmitter clock and receiver clock. The inventive expanded longtraining sequence is usable by 802.11a or 802.11g systems only if thevalues at sub-carriers −26 to +26 are identical to those of the currentlong training sequence used in 802.11a and 802.11g systems. As such, theinvention utilized the same +1 or −1 binary phase shift key (BPSK)encoding for each new sub-carrier and the long training sequence of802.11a or 802.11g systems is maintained in the present invention.

In a first embodiment of the invention, the expanded long trainingsequence is implemented in 56 active sub-carriers including sub-carriers−28 to +28. In another embodiment, an expanded long training sequence isimplemented using 63 active sub-carriers, i.e., all of the activesub-carriers (−32 to +31) except the 0-index sub-carrier which is set to0. In both embodiments of the invention, orthogonality is not affected,since a 64-point orthogonal transform is used to generate thetime-domain sequence. Additionally, the output of an autocorrelator forcomputing the carrier frequency offset is not affected by the extrasub-carriers.

FIG. 2 illustrates a schematic block diagram of a processor that isconfigured to generate an expanded long training sequence. Processor 200includes a symbol mapper 202, a frequency domain window 204, a signalgenerating circuit 205, an inverse fast Fourier transform (IFFT) module206, a serial to parallel module 208, a digital transmit filter and/ortime domain window module 210, and digital to analog converters (D/A)212. For an expanded long training sequence, symbol mapper 202 generatessymbols from the coded bits for each of the 64 subcarriers of an OFDMsequence. Frequency domain window 204 applies a weighting factor on eachsubcarrier. Signal generating circuit 205 generates the expanded longtraining sequence and if 56 active sub-carriers are being used, signalgenerating circuit generates the expanded long training sequence andstores the expanded long training sequence in sub-carriers −28 to +28.If 63 active sub-carriers are being used, signal generating circuitgenerates the expanded long training sequence and stores the expandedlong training sequence in sub-carriers −32 to +32 i.e., all of theactive sub-carriers (−32 to +31) except the 0-index sub-carrier which isset to 0. The inventive long training sequence is inputted into anInverse Fourier Transform 206. The invention uses the same +1 or −1 BPSKencoding for each new sub-carrier. Inverse Fourier Transform 206 may bean inverse Fast Fourier Transform (IFFT) or Inverse Discrete FourierTransform (IFDT). Inverse Fourier Transform 206 processes the longtraining sequence from signal generating circuit 205 and thereafterproduces an optimal expanded long training sequence with a minimalpeak-to-average power ratio. The optimal expanded long training sequencemay be used in either 56 active sub-carriers or 63 active subscribers.Serial to parallel module 208 converts the serial time domain signalsinto parallel time domain signals that are subsequently filtered andconverted to analog signals via the D/A.

FIG. 3 is a schematic block diagram of a processor that is configured toprocess an expanded long training sequence. Processor 300 includes asymbol demapper 302, a frequency domain window 304, a fast Fouriertransform (FFT) module 306, a parallel to serial module 308, a digitalreceiver filter and/or time domain window module 310, and analog todigital converters (A/D) 312. A/D converters 312 convert the sequenceinto digital signals that are filtered via digital receiver filter 310.Parallel to serial module 308 converts the digital time domain signalsinto a plurality of serial time domain signals. FFT module 306 convertsthe serial time domain signals into frequency domain signals. Frequencydomain window 304 applies a weighting factor on each frequency domainsignal. Symbol demapper 302 generates the coded bits from each of the 64subcarriers of an OFDM sequence received from the frequency domainwindow.

FIG. 4 illustrates the long training sequence with a minimumpeak-to-average power ratio that is used in 56 active sub-carriers. Outof the 16 possibilities for the four new sub-carrier positions, thesequence illustrated in FIG. 4 has the minimum peak-to-average powerratio, i.e., a peak-to-average power ratio of 3.6 dB.

FIG. 5 illustrates the long training sequence with a minimumpeak-to-average power ratio that is used in 63 active sub-carriers. Outof the 2048 possibilities for the eleven new sub-carrier positions, thesequence illustrated in FIG. 5 has the minimum peak-to-average powerratio, i.e., a peak-to-average power ratio of 3.6 dB.

It should be appreciated by one skilled in art, that the presentinvention may be utilized in any device that implements the OFDMencoding scheme. The foregoing description has been directed to specificembodiments of this invention. It will be apparent, however, that othervariations and modifications may be made to the described embodiments,with the attainment of some or all of their advantages. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A wireless communications device, comprising: aprocessor configured to provide a signal generator and an InverseFourier Transformer, wherein the signal generator generates an extendedlong training sequence, wherein the Inverse Fourier Transformerprocesses the extended long training sequence from the signal generatorand provides an optimal extended long training sequence with a minimalpeak-to-average ratio, wherein at least the optimal extended longtraining sequence is carried by a greater number of subcarriers than astandard wireless networking configuration for an Orthogonal FrequencyDivision Multiplexing scheme, wherein the wireless communications deviceregisters with an access point or a base station.
 2. The wirelesscommunications device according to claim 1, wherein at least the optimalextended long training sequence is carried by at least 56 activesub-carriers.
 3. The wireless communications device according to claim2, wherein the at least 56 active sub-carriers correspond to at leastindexed sub-carriers −28 to +28.
 4. The wireless communications deviceaccording to claim 2, wherein the optimal extended long trainingsequence has a minimum peak-to-average power ratio of 3.6 dB.
 5. Thewireless communications device according to claim 1, wherein at leastthe optimal extended long training sequence is carried by at least 63active sub-carriers.
 6. The wireless communications device according toclaim 5, wherein the at least 63 active sub-carriers correspond to atleast indexed sub-carriers −32 to +31.
 7. The wireless communicationsdevice according to claim 5, wherein the optimal extended long trainingsequence has a minimum peak-to-average power ratio of 3.6 dB.
 8. Thewireless communications device according to claim 1, wherein a binaryphase shift key encoding is used for each sub-carrier above the +26indexed sub-carrier and below the −26 indexed sub-carrier.
 9. Thewireless communications device according to claim 1, wherein the InverseFourier Transformer is configured as an Inverse Fast Fourier Transformeror an Inverse Discrete Fourier Transformer.
 10. The wirelesscommunications device according to claim 1, wherein the wirelesscommunications device is one or more of the following: a personaldigital assistant, a laptop computer, a personal computer and a cellularphone.
 11. The wireless communications device according to claim 1,wherein the wireless communications device comprises a wireless mobilecommunications device.
 12. The wireless communications device accordingto claim 1, wherein the wireless communications device comprises one ormore of the following: an access point and a base station.
 13. Thewireless communications device according to claim 1, wherein thewireless communications device is backwards compatible with legacywireless local area network devices.
 14. A wireless cellular device,comprising: a processor configured to provide a signal generator and anInverse Fourier Transformer, wherein the signal generator generates anextended long training sequence, wherein the Inverse Fourier Transformerprocesses the extended long training sequence from the signal generatorand provides an optimal extended long training sequence with a minimalpeak-to-average ratio, and wherein at least the optimal extended longtraining sequence is carried by a greater number of subcarriers than astandard wireless networking configuration for an Orthogonal FrequencyDivision Multiplexing scheme, wherein the wireless cellular deviceregisters with an access point or a base station.
 15. The wirelesscellular device according to claim 14, wherein the wireless cellulardevice is a mobile wireless cellular device.
 16. The wirelesscommunications device according to claim 15, wherein at least theoptimal extended long training sequence is carried by at least 56 activesub-carriers.
 17. The wireless communications device according to claim16, wherein the at least 56 active sub-carriers correspond to at leastindexed sub-carriers −28 to +28.
 18. A mobile cellular phone,comprising: a processor configured to provide a signal generator and anInverse Fourier Transformer, wherein the signal generator generates anextended long training sequence, wherein the Inverse Fourier Transformerprocesses the extended long training sequence from the signal generatorand provides an optimal extended long training sequence with a minimalpeak-to-average ratio, and wherein at least the optimal extended longtraining sequence is carried by a greater number of subcarriers than astandard wireless networking configuration for an Orthogonal FrequencyDivision Multiplexing scheme, wherein the mobile cellular phoneregisters with an access point or a base station.
 19. The mobilecellular phone according to claim 18, wherein at least the optimalextended long training sequence is carried by at least 56 activesub-carriers.
 20. The mobile cellular phone according to claim 19,wherein the at least 56 active sub-carriers correspond to at leastindexed sub-carriers −28 to +28.